The present invention relates to a sheet feeding apparatus for separating a sheet one by one from a stack of sheets placed on a stacker and feeding the sheet to a processing platen for processing the sheet such as reading or printing an image, and relates to an image reading apparatus with the sheet feeding apparatus. The present invention also relates to a method of detecting a double feed of a plurality of sheets during a sheet feeding process.
A sheet feeding apparatus is generally known as an apparatus for sequentially feeding a sheet one by one from a stack of sheets placed on a stacker to a processing platen in a printer, a copying machine, a scanner, or the like. When the sheet feeding apparatus attempts to separate one sheet at a time from a stack of sheets on a stacker and feed the sheet to a processing platen, two or more sheets may overlap with each other during the feeding process. In such a case, the sheets are fed to the processing platen without being properly separated from each other, which is called a double feed. When the double feed occurs, a wrong processing may be performed at the processing platen. Accordingly, if the sheets are not separated accurately, it is necessary to detect the double feed before the sheets reach the processing platen to stop the processing or cancel processed data such as reading.
A conventional method of detecting the double feed of sheets includes an ultrasonic sensor, a photo-sensor, or the like for detecting attenuation in an ultrasonic wave or an intensity of light passing through the sheet, thereby determining whether there is a single sheet.
Japanese Patent Publication (Kokai) No. 10-257595 discloses an ultrasonic sensor for detecting a sheet during transportation. A conventional ultrasonic sensor includes a piezoelectric oscillation plate such as piezoelectric ceramic at a wave transmission side and a similar oscillation plate at a wave reception side. A pulse voltage with a predetermined cycle is applied to the piezoelectric oscillation plate at the wave transmission side to generate oscillation, thereby transmitting an ultrasonic wave. The piezoelectric oscillation plate at the wave reception side receives the ultrasonic wave and converts to an electrical signal. Electric energy applied to the piezoelectric oscillation plate (wave transmission element) at the wave transmission side is compared with electric energy generated at the piezoelectric oscillation plate (wave reception element) at the wave reception side, thereby determining whether there is a single sheet.
When the double feed is detected with such an ultrasonic sensor, it is necessary to accurately measure the ultrasonic energy (electric energy output from the wave reception element) attenuated through the sheet between the wave transmission element and the wave reception element. U.S. Pat. No. 6,212,130 discloses a conventional structure in which a wave transmission element and a wave reception element are arranged opposite to each other with a predetermined angle relative to a surface of a sheet. With this structure, it is possible to prevent an ultrasonic wave transmitted from the wave transmission element from reflecting at the sheet surface and interfering.
Japanese Utility Model Publication (Kokai) No. 06-49567 proposes a structure in which a wave transmission element and a wave reception element are arranged opposite to each other between a downstream roller and an upstream roller arranged with a predetermined distance in between, thereby detecting the double feed while a sheet is in a stable condition. More specifically, with the structure, the double feed is detected while the downstream and upstream rollers nip the sheet in a straight position during transportation. Accordingly, it is possible to accurately detect the double feed since a leading end or a trailing end of the sheet is not curved or does not oscillate vertically.
When the double feed is detected with the conventional ultrasonic sensor or optical sensor, an instantaneous wave such as a burst wave is used to measure an amount of the wave transmitted through a sheet. With this method, when the sheets are overlapped and shifted in a feeding direction, it is difficult to accurately detect the double feed. Further, an image reading apparatus handles a wide variety of sheets in terms of quality, weight, and size. In such a case, it is necessary to provide a detection area with a predetermined length along a sheet transport direction, so that it is possible to apply a burst wave or a steady-state wave several times and detected signals are averaged for detecting the double feed.
As described above, Japanese Utility Model Publication (Kokai) No. 06-49567 discloses the structure in which the transport rollers are arranged separately with a predetermined distance in between in the feeding direction, so that the double feed is detected while the transport rollers nip a sheet. In Japanese Utility Model Publication (Kokai) No. 06-49567, a sheet sensor detects that the transport rollers at a downstream side nip a leading end of the sheet, and the double feed is detected based on a detection signal in this state. However, when the sheets are overlapped and shifted in the feeding direction, the double feed may be detected while a trailing end of the sheet leaves the transport rollers at a downstream side and is flapping, thereby causing a false detection.
In view of the problems mentioned above, an object of the present invention to provide a sheet feeding apparatus in which at least two transport devices are arranged at an upstream side and a downstream side in a feeding direction to obtain a specific measurement area in between while nipping and transporting a sheet. When the double feed is detected, the transport device at the upstream side securely nips a trailing end of the sheet.
Another object of the present invention is to provide a sheet feeding apparatus and a method of accurately detecting the double feed in which even when a sheet does not have a specific length, it is possible to accurately detect the double feed including a case that the sheets are overlapped and shifted in a feeding direction.
Further objects and advantages of the invention will be apparent from the following description of the invention.